Valves are used in a multitude of applications to control the passage of fluids. In some applications, such as biomedical equipment and chemical process control equipment, human lives can be endangered if a valve fails during operation. Failures may include a failure to actuate, a failure to deactuate or a failure caused by leaks in and around the valve.
A number of techniques are known in the prior art to monitor the operation of a valve. One technique is to include an electromechanical sensor in the valve housing to detect the movement of the flapper between its open and closed positions. Such electromechanical sensors often comprise switches that are operated by the movement of the valve flapper and produce an on or off signal indicating the position of the valve flapper and correspondingly the state of the valve.
Electromechanical valve monitoring systems suffer from a number of drawbacks. One is the poor reliability of switch mechanisms. Virtually all switches have a finite lifetime. When the switch fails, operation of the valve monitor ceases. There is generally no method for detecting switch failure except by making frequent redundant checks of the valve's state.
Another drawback to electromechanical monitoring systems is that they generally do not indicate the relative degree of valve closure: the sensing switch is either on or off, without any provision for indicating a partial opening or closure. In many applications, any opening of a valve, regardless of how minute, can be critical to the operation of a system.
Finally, electromechanical monitoring systems suffer in that they cannot detect leaks through or around a valve flapper, by which fluid from the inlet might communicate with fluid in the outlet.
Examples of electromechanical valve monitors are shown in U.S. Pat. Nos. 4,340,086 and 4,376,450.
In another class of valve monitors, a visual indication of the valve status is produced. In such monitors, an indicator external to the valve housing changes position with the valve flapper, signaling to the operator the state of the valve. Although some of the reliability problems associated with other monitoring systems are eliminated by this approach, it nonetheless suffers from poor resolution of intermediate flapper positions and is unable to detect leaks around the valve.
To detect leaks around a valve, it has generally been necessary to conduct a static pressure test. In such a test, a pressure is applied to the valve in its closed state and the decay of this pressure over time is monitored. The rate at which the pressure decays indicates the relative leak rate of the valve.
The static pressure test suffers from a number of obvious drawbacks. First is that several extraneous components, not generally included in the system being tested, must be provided in order to apply the static pressure to the valve and to measure the resultant pressure decay. Another drawback is that the test is conducted at a time and under a set of conditions isolated from the valve's normal operation. The method cannot be used to check leakage during actual operation. Finally, the test circumstances rarely duplicate actual dynamic operating conditions and thus are a poor predictor of valve performance during normal operation.
Yet another technique for monitoring valve operation is to monitor with a flow meter the rate of fluid flow through the valve. This technique, however, relies on the proposition that the pressure driving the fluid is constant, so that any variation in flow must be caused by changes in the valve. The flow meter technique is also ill-suited to detect the minute fluid flows associated with most valve leaks.
From the foregoing it will be recognized that the current state of the art in valve monitors is deficient in several respects. There is no convenient technique for determining leakage through a closed valve while it is in operation. Similarly, there is no convenient technique for monitoring the status of a valve in its partially opened or closed state. Finally, prior art techniques suffer from a host of reliability problems that make them fraught with risk for use in sensitive biomedical and chemical process control applications.
Accordingly, a need remains for an improved method and apparatus for monitoring the operation of a valve.